Acorns: A Fickle Crop?

In so many ways, this has been an interesting year! We are fortunate to live in Pennsylvania with sylvan landscapes where we can distance ourselves among the trees. Some folks are observing early acorn drop. This raises questions about this fickle crop and what may explain what keen observers are seeing.

Oaks are economically important tree species in Pennsylvania and across the East. Acorns, or mast - a word derived from old English which means forest food, are important to wildlife. Oak-borne mast production varies from year to year. Much research has sought to predict masting years when big acorn crops occur and shed light on what leads to poor years or early acorn abortion (or abscission) like you may have seen this season.

This story starts by recognizing that oaks separate into two groups popularly referred to as red and white. Learning to identify the difference between the two groups is easy. Red oaks, for example Northern red oak, pin oak, scarlet oak, and black oak, have small “bristles" on the lobes and tips of their leaves. White oaks, for example white oak and rock, or chestnut, oak, lack these bristles and have rounded leaf lobes. There are other differences that are more difficult to recognize such as acorn structure and wood anatomy.

Both red and white oaks produce female and male flowers on the same tree, unlike ash which has male and female trees. The process of producing an acorn starts late in the growing year when the male flowers form as the tree's growth slows toward the end of summer. That is the end of the first year (year 1) in the process. Then, in the second year (year 2), as the tree comes out of dormancy, female flowers form in the axil of the leaf stem and the twig and remain dormant. As the spring leaves begin to unfold, the male flowers emerge and are very apparent as rather-long, drooping, greenish-yellow catkins. These appear about two weeks before the much smaller female flowers emerge.

For the white oaks as the male and female flowers emerge in year 2 as described above, pollination and fertilization should happen. The pollen from the male flower, which is wind-disseminated, lands on the style which is part of the female flower. When this happens, the pollen initiates the development of a pollen tube that transfers male cells into the ovule to complete fertilization and the process of acorn formation should start in earnest. For this to happen, it is ideal that to have warm days and cool nights. If temperatures are not right, fertilization may fail, and the female flowers will abort, which results in low acorn initiation. Alternatively, if temperatures become too hot or drought conditions occur, white oak acorns may abort, which is likely apparent in mid-June to mid- to late-July; perhaps that is the reason for reported early acorn drop this year. The other big threat to white oak acorns is late spring frosts, which happened this year, and would again remove the fertilized flowers.

Acorn production for the red oaks is more extended. As described earlier, year 2 begins with emergence of the male and female flowers and pollen encounters female styles and the pollen tubes form and extend toward the ovule; however, the male cell journey stops at that point until the next year (year 3). Looking closely at the intersection of the leaf stem and the twig on current year's growth, there is a small structure that looks similar to the familiar unground clove spice head. In the spring of year 3, the male cell will complete its journey and complete fertilization and the embryonic acorn will mature in the third year. As with white oaks, weather plays a role in the successful conclusion of this process for red oaks. At least one study has found that weather events explain the loss of 55% of black oak acorns and up to 89% of the red oak acorns. In a hierarchy of impacts, summer drought, followed by spring temperatures, as with white oaks, and year 2 spring frost dates are most important. There is some suggestion that spring rains when pollen release occurs is another big influence on the setting of the embryonic acorns in the second year. Again, in some parts of Pennsylvania, this has not been a friendly year for the formation of red oak group acorns. Depending upon where you are, this spring there were frosts, above average rainfall, and drought. The first two issues may mean fewer acorns in 2021 and the drought may abort acorns in 2020.

Long-term studies to unravel the complexity of acorn production are difficult to conduct and are not common. Researchers have learned that under ideal conditions most of Pennsylvania's oak species should create synchronized masting crops across landscapes with some consistency. Studies have found that big acorn crops follow years with sparse crops. Ideally, black oak will have good crops every other year, Northern red oak in 4-year cycles, and white oak in 3-year cycles.

The reasons behind synchronized acorn crops is unclear. Some researchers suggest that the entire process can deplete tree nutrient reserves and it takes time to recover. When trees experience stress, such as drought, there is early acorn abscission, which likely reflects reserved resource responses. Other researchers suggest that synchronization is a response to seed predation – a response designed to ensure acorns occur when insects, animals, and birds are at low densities. Clearly, though, weather is a big factor.

Because acorns are important to wildlife and the continuation of our oak forests as our climate changes, it is important to manage forests with oak species to retain them as part of the tree mix. From a wildlife perspective, a mix of oak species from the red and white oak groups is important to help assure at least some mast occurs nearly every year. Not all oak trees in a given species are great producers. For example, there is evidence that some trees never or seldom produce acorns. On the flipside, some trees seemingly do reasonably well every year. Further, there is evidence that forest grown trees with wide crowns yield more fruit.

There is one more important difference between the red and white oak groups – they germinate at different times. White oaks germinate in the autumn sometimes before they even fall from the tree. Look for them in the forest and you will often see the root radical sprouting from the acorn. Because they contain less tannins than red oak acorns this adaptation helps protect them from herbivores eager for the mast. After falling red oaks overwinter and germinate in the spring. They have higher tannin levels and are more bitter tasting. As a result, they are often eaten later in the winter. Importantly they really benefit from leaf cover to protect them from drying out over winter. Maybe you have noticed that red oak acorns fall before the leaves come down?

So, as you walk in forests, think about how hard oak trees work to produce their acorns. Consider the value they provide for wildlife. And, recall that from acorns, mighty oaks grow!

Written by Jim Finley, Professor Emeritus, Forest Resources Management, Center for Private Forests at Penn State.